In conventional earth bore hole drilling apparatus, the drill head is axially stabilized by the load required to destroy rock. Blade type cutting structures require very little axial load to bite into the formations drilled and axial control or stabilization is almost absent. Blade type bits cutting soft formations encounter few problems because torque does not increase drastically as advance per turn increases. Similarly, shallow drilling with blade type cutters encounters few problems because short suspended drill strings are, axially, quite rigid and advance per turn can be controlled by the upper end of the drill string. On long drill strings, there is a lower axial sping rate in the drill string and more stick-slip problems arise and axial stability decreases.
Hole openers are subject to damagae from reduction of axial stability. It is well known that diamond bits used to ream holes experience wide fluctuations of drilling torque and bits so used are often damaged in the gage region. On drilling turbines, diamond bits used to ream holes cause severe speed fluctuations and considerable damage. Any attempt to use poly-crystalline diamond cutters on hole openers must be structured with the damage history in view. Blade type cutting devices fitted with poly-crystalline compacts still behave as a blade type cutter as previously described. The use of the existing, or pilot, hole for radial stabilization assumes confidence on the remaining integrity of the hole, devoid of bridges, fractures and loosened inclusions. At best, only radial stability may be taken for granted.